Mass flow controllers have been widely used as flow rate control apparatuses for the gas supply system in semiconductor manufacturing plants. Recent years have seen the development of pressure-type flow rate control apparatuses that are replacing the mass flow controllers. Among the newly developed pressure-type control apparatuses are those disclosed in unexamined Japanese patent publications Nos. 8-335117 and 8-338546.
FIG. 11 shows the pressure-type flow rate control apparatus which the inventors disclosed earlier in the above-mentioned unexamined Japanese patent publication No. 8-338546. The operating principle of that pressure-type flow rate control apparatus is this: The flow rate Qc of fluid on the downstream side of the orifice is calculated with an equation Qc=KP1 (K: constant), with the ratio P2/P1 of pressure P2 at the downstream side of an orifice 5 to pressure P1 of the upstream side held below the gas critical pressure ratio. The difference between the calculated flow rate Qc and the set flow rate Qs is input in a valve drive 3 for a control value 2 as control signal Qy to regulate the degree of opening of the control valve 2 for adjusting the pressure P1 upstream of an orifice 5, such that the calculated flow rate Qc=the set flow rate Qs (that is, the control signal Qy=0) is achieved. Thus, the flow rate on the downstream side of the orifice is regulated to the aforesaid set flow rate Qs.
Referring to FIG. 11, the reference number 1 indicates a pressure-type flow rate control apparatus; 2, a control valve; 3, a control valve drive; 4, a pressure detector; 5, an orifice; 7, a control unit; 7a, a temperature correction circuit; 7b, a flow rate calculation circuit; 7c, a comparison circuit; 7d, an amplification circuit; 21a and 21b, amplification circuits; 22a and 22b, A-D conversion circuits; 24, an inverted amplifier; 25, a valve; Qc, signal for calculated flow rate; Qs, signal for set flow rate; and Qy, control signal (Qc-Qs).
The aforesaid pressure-type flow control system permits setting the flow rate Q on the downstream side of the orifice at a desired level with high precision through adjustment of the pressure P1 on the upstream side of the orifice by actuating the control valve 2. Thus, the apparatus is a highly effective tool in practice.
However, the problem with that pressure-type flow rate control apparatus is that because the orifice 5 is fixed in diameter, the application is limited to a specific range of flow rates and no switch-over in the flow rate ranges is possible.
To make the switch-over possible, it is necessary to so design the orifice 5 as to be readily replaceable, and to prepare a plurality of orifices 5 with different bores or calibers ready for use. But a problem is that non-uniformity in precision of processing of those orifices 5 leads directly to errors in flow rate control.
Preparing a plurality of orifices with different bores present problems such as lack of economy and poor control precision.
Also, in a flow rate controller of a fixed flow rate type using the so-called sonic velocity nozzle (or orifice), sectional area-variable nozzles or orifices have been developed for permitting a change in the flow rate range, and disclosed in unexamined Japanese utility model publication No. 56-41210 and examined Japanese utility model publication No. 60-42332.
However, these sectional area-variable orifices are all those with mechanisms similar to needle-type valves and are inevitably accompanied by many dead spaces in the fluid flow path. That makes complete gas switching or replacement difficult and causes much dust. For this reason, those sectional area-variable orifices are not very suitable for use in the gas supply system in semiconductor manufacturing facilities.